Injection Level Dependent Lifetime Research Articles

Extracting dielectric fixed charge density on highly doped crystalline-silicon surfaces using photoconductance measurements

A novel method for the extraction of fixed interface charge, Qf, and the surface recombination parameters, Sn0 and Sp0, from the injection-level dependent effective minority carrier lifetime measurements is presented. Unlike conventional capacitance-voltage measurements, this technique can be applied to highly doped surfaces provided the surface carrier concentration transitions into strong depletion or inversion with increased carrier injection. By simulating the injection level dependent Auger-corrected inverse lifetime curve of symmetrically passivated and diffused samples after sequential annealing and corona charging, it was revealed that Qf, Sn0, and Sp0 have unique signatures. Therefore, these important electronic parameters, in some instances, can independently be resolved. Furthermore, it was shown that this non-linear lifetime behaviour is exhibited on both p-type and n-type diffused inverted surfaces, by demonstrating the approach with phosphorous diffused n+pn+ structures and boron diffused p+np+ structures passivated with aluminium oxide (AlOx) and silicon nitride, respectively (SiNx). The results show that the approximation of a mid-gap Shockley-Read-Hall defect level with equal capture cross sections is able to, in the samples studied in this work, reproduce the observed injection level dependent lifetime behaviour.

Improved understanding of the recombination rate at inverted p+ silicon surfaces

The effect of positive fixed charge on the recombination rate at SiNx-passivated p+ surfaces is studied in this work. It is shown that a high positive fixed charge on a low defect density, passivated doped surface can result in a near injection level independent lifetime in a certain injection level range. This behaviour is modelled with advanced computer simulations using Sentaurus TCAD, which replicates the measurements conditions during a photoconductance based effective minority carrier lifetime measurement. The resulting simulations show that the shape of the injection level dependent lifetime is a result of the surface recombination rate, which is non-linear due to the surfaces moving into inversion with increasing injection level. As a result, the surface recombination rate switches from being limited by electrons to holes. Equations describing the surface saturation current density, J0s, during this regime are also derived in this work.

Carrier Lifetime in Liquid-phase Crystallized Silicon on Glass

Liquid-phase crystallized silicon on glass (LPCSG) presents a promising material to fabricate high quality silicon thin films, e.g. for solar cells and modules. Barrier layers and a doped amorphous silicon layer are deposited on the glass substrate followed by crystallization with a line focus laser beam. In this paper we introduce injection level dependent lifetime measurements generated by the quasi steady-state photoconductance decay method (QSSPC) to characterize LPCSG absorbers. This contactless method allows a determination of the LPCSG absorber quality already at an early stage of solar cell fabrication, and provides a monitoring of the absorber quality during the solar cell fabrication steps. We found minority carrier lifetimes higher than 200ns in our layers (e.g. n-type absorber with ND=2x1015cm-3) indicating a surface recombination velocity SBL<3000cm/s at the barrier layer/Si interface.

Boron-Oxygen Defect Formation Rates and Activity at Elevated Temperatures

In this work the dependence of the slow boron-oxygen defect formation rate on excess carrier density is examined in p-type Cz silicon. In order to examine behavior at elevated temperatures simple models are developed to simulate the injection-level dependent lifetime of samples at a range of temperatures and active defect concentrations. These models are then verified against experimental data. Based on these models it is possible to clearly observe a quadratic dependence of defect formation rate upon total hole concentration over a range of temperatures. The implications of a hole (and hence excess carrier (Δn)) dependent defect formation rate, combined with the temperature dependence of defect activity are then discussed. It is demonstrated how a dependence of formation rate upon hole concentration increases the rate of defect formation and mitigation of carrier-induced degradation in samples with reduced saturation current density during anneals at elevated temperatures and illumination intensities.

Impact of the injection‐level‐dependent lifetime on Voc, FF, ideality m, J02, and the dim light response in a commercial PERC cell

AbstractThe injection‐level‐dependent (ILD) lifetime of the silicon wafer impacts many characteristics of the final photovoltaic cell. While efficiency is commonly understood to be impacted by the silicon bulk lifetime (at the maximum power point injection level), this work demonstrates the wide ranging impacts of ILD lifetime on the Voc, the fill factor (FF), the diode ideality factor m, and the dim light response. Instead of a two‐diode model, we utilize a boundary + ILD bulk lifetime model to analyze a commercial passivated emitter rear contact (PERC) cell featuring an AlOx dielectric rear passivation. The ILD lifetime is directly measured and used to calculate the bulk recombination current across injection levels. With this boundary + ILD lifetime model, we demonstrate the role of the ILD lifetime on many cell parameters in this PERC cell. For most high efficiency commercial p‐type monocrystalline solar cells, the typically lower bulk lifetime at the maximum power point versus the lifetime at the open circuit point reduces the measured FF and pseudo‐FF. This work illustrates that for a commercial PERC cell with AlOx rear passivation, the ILD lifetime is the primary mechanism behind reduced FF, ideality factors greater than 1, and the source of the J02 term in the two‐diode model. The crucial implications of this work are not only to better understand commercial PERC cell loss mechanisms but also to encourage a focus on different metrics in cell diagnostics. One such metric is the Voc at 0.1 or 0.05 suns. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Fill Factor Limitation of Silicon Heterojunction Solar Cells by Junction Recombination

In this work we experimentally investigate the influence of the chemical and field effect passivation of a-Si:H films on the junction recombination at maximum power point conditions of silicon heterojunction solar cells. By simulating the charge of doped a-Si:H films with external corona charges, chemical and field effect passivation are investigated independent of each other. If we apply a negative corona charge to a lifetime test structure inversion conditions are obtained which corresponds to a p/n junction. With positive charges we can create accumulation i.e. a high/low junction (for an n-type wafer). From injection level dependent lifetime measurements the implied solar cell parameters iVoc and iFF are extracted. Especially the implied fill factor is influenced by the recombination characteristics at the injection level at maximum power point conditions. It is shown that under inversion conditions the implied solar cell parameters are lower compared to accumulation conditions. The main reason for this seems to be the asymmetry in defect capture cross sections for electrons and holes. As this defect characteristic at the c-Si/a-Si:H interface cannot be altered, a considerable reduction of the defect density is required to further decrease junction recombination at M PP conditions and thus increase the FF of SHJ cells.

IR-study of a-SiC x:H and a-SiC xN y:H films for c-Si surface passivation

Amorphous intrinsic silicon carbide (a-SiC x :H(i)) films and amorphous silicon carbonitride (SiC x N y :H) films have been deposited by plasma enhanced chemical vapor deposition from CH 4/SiH 4/(N 2) plasma on n-type (1.5 Ω cm) crystalline silicon (c-Si). These films have evidenced excellent surface passivation properties with a best result for the surface recombination velocity S eff≈2–3 cm s −1 for amorphous SiC x N y :H films, which is the lowest reported value for n-type c-Si with resistivity of 1.5 Ω cm so far. IR transmission spectroscopy was performed to get more detailed information about the structural composition of the films. Integrated absorption of the bands typical for SiC and SiN bonds reveals that N 2 addition to the plasma leads to nitrogen incorporation into the films but also enhances the incorporation of carbon. From IR analysis it is found that the characteristic minimum of the S eff for a certain CH 4/SiH 4 gas flow ratio is not correlated with the bulk properties of the amorphous films. However, first analysis of injection level dependent lifetime measurements indicates that the minimum in S eff is related to good interface properties of the silicon carbon alloy deposited in the composition range of x≈0.2.

Capture cross sections of the acceptor level of iron–boron pairs in p-type silicon by injection-level dependent lifetime measurements

Injection-level dependent recombination lifetime measurements of iron-diffused, boron-doped silicon wafers of different resistivities are used to determine the electron and hole capture cross sections of the acceptor level of iron–boron pairs in silicon. The relative populations of iron–boron pairs and interstitial iron were varied by exposing the samples to different levels of illumination prior to lifetime measurements. The components of the effective lifetime due to interstitial iron and iron–boron pairs were then modeled with Shockley–Read–Hall statistics. By forcing the sum of the modeled iron–boron and interstitial iron concentrations to equal the implanted iron dose, in conjunction with the strong dependence of the shape of the lifetime curves on dopant density, the electron and hole capture cross sections of the acceptor level of iron–boron pairs have been determined as (3±2)×10−14 cm−2 and (2±1)×10−15 cm−2.

ON THE NONLINEAR PROPERTIES OF THE PHOTO-MAGNETOELECTRIC EFFECT IN SEMICONDUCTORS

The equation of continuity of the charge carriers through a semiconductor was reduced to a second order non-linear differential equation, by using the injection-leveldependent lifetime which was derived from Shockley-Read statistics. The general solution was found using two-parameters perturbation method. The PME short-circuit current Isc and the photoconductance ΔG in a semiconductor are calculated in the second order approximation. The nonlinear properties of the photomagnetoelectric effect in the case of large signal are disclosed, experimental results are better fitted by the theory.